As solid-state imaging devices (image sensors) using photoelectric conversion elements which detect light to generate electric charges, CMOS (complementary metal oxide semiconductor) image sensors have been put into practical use. CMOS image sensors are being widely applied as parts of digital cameras, video cameras, monitoring cameras, medical endoscopes, personal computers (PC), mobile phones, and other portable terminal devices (mobile devices) and various other types of electronic apparatuses.
A CMOS image sensor has a floating diffusion (FD) amplifier having, for each pixel, a photodiode (photoelectric conversion element) and floating diffusion layer. For readout, the mainstream type is the column parallel output type that selects a certain row in a pixel array and simultaneously reads the pixels out to a column output direction.
Each pixel in a CMOS image sensor basically includes as active elements, for example, for one photodiode, four elements of a transfer element constituted by a transfer transistor, a reset element constituted by a reset transistor, a source-follower element (amplification element) constituted by a source-follower transistor, and a selection element constituted by a selection transistor. Further, each pixel may be provided with an overflow gate (overflow transistor) for discharging an overflow charge overflowing from the photodiode in a storage period of the photodiode.
The transfer transistor is held in a non-conductive state during a charge accumulation period of the photodiode and is supplied with a driving signal at its gate and held in a conductive state and transfers the charge photo-electrically converted in the photodiode to the floating diffusion FD during a transfer period for transferring the accumulated (stored) charge of the photodiode to the floating diffusion FD.
The reset transistor resets the potential of the floating diffusion FD to the potential of the power supply line when a reset signal is given to its gate.
The floating diffusion FD is connected the gate of the source-follower transistor. The source-follower transistor is connected through the selection transistor to a vertical signal line and configures a source-follower together with a constant current source in a load circuit outside of the pixel portion. Further, a control signal (address signal or select signal) is given to the gate of the selection transistor, whereupon the selection transistor turns ON. When the selection transistor becomes ON, the source-follower transistor amplifies the potential of the floating diffusion FD and outputs a voltage in accordance with that potential to the vertical signal line. Through the vertical signal line, the voltage output from the pixel is output to a pixel signal readout circuit constituted by a column parallel processing part. In the column parallel processing, the image data is for example converted from an analog signal to digital signal, transferred to a later stage signal processing part, and subjected to predetermined image signal processing to obtain a desired image.
As explained above, in a CMOS image sensor, electrons which are generated by photo-electric conversion by slight light are converted to voltage by a very small capacitance and further are output by using a source-follower transistor having a very small area. For this reason, it is necessary to remove very small noise such as noise generated when resetting the capacitance or manufacturing fluctuations of the transistors, therefore the difference between the reset level and the luminance level (signal level) for each pixel is output. In this way, in a CMOS image sensor, by outputting the difference between the reset level and the luminance level for each pixel, the reset noise and threshold value fluctuations are removed, therefore signals of a few electrons can be detected. The operation of detecting this difference is a technique called “correlated double sampling (CDS)” and is widely used. All of the pixels arranged in an array state are sequentially read out by CDS and 1 frame's worth of usual pixel data is output.
In this regard, in the solid-state imaging device (image sensor) explained above, basically owners of various types of electronic apparatuses or users permitted to use the same can easily reproduce captured image data and view the images. In current solid-state imaging devices, however, even in a case where the captured image data is data concerned with personal secrets, it can be easily reproduced, therefore there is the disadvantage that unauthorized use or tampering, forgery, etc. of an image end up being easy. It is possible to secure uniform secrecy by encrypting using a unique key. In actual circumstances, however, it is difficult to secure tamper resistance (difficulty of breaking) of a unique key.
Therefore, in order to solve these problems, there has been proposed a solid-state imaging device (image sensor) capable of securing tamper resistance of a unique key and consequently capable of preventing tampering and forgery of an image (see PLT 1). This PLT 1 describes a method for extracting so-called “fingerprint information” for each chip of a CMOS image sensor.
The solid-state imaging device (image sensor) disclosed in PLT 1 basically has a pixel portion in which a plurality of pixels each including a photodiode are arranged in rows and columns, a reading part for reading pixel signals from the pixel portion, and a key generation part which generates a unique key by using at least one of fingerprint information constituted by fluctuation information of pixels and fluctuation information of the reading part.